1. Field of the Invention
The present invention relates to a warning circuit, and particularly to a weak, low-voltage battery warning circuit.
2. General Background
A CMOS random access memory (RAM) located on a motherboard of a computer is used to maintain data on hardware configuration of a current system and on settings and parameters such as date, time, hard disk parameters and so on. The CMOS RAM is made using complementary metal oxide semiconductor (CMOS) technology. Typically. chipset manufacturers integrate the CMOS RAM into a real-time clock in an input/output (I/O) Controller Hub. A power supply circuit connected to the real-time clock provides power supply to the CMOS RAM for sustaining the data that the CMOS RAM keeps. Referring to FIG. 3, the power supply circuit 3 of the real-time clock includes an or-gate circuit using diodes D6, D7. When the computer is disconnected from an outer power supply, a battery 5 set on the motherboard provides power to the real-time clock 7. When the computer is connected to the outer power supply, the battery 5 does not provide power to the real-time clock 7 any more and a 3.3V standby voltage from a computer power supply is provided to the real-time clock 7.
After the computer has been left unused for some time and then powered up, an error message may be displayed such as “CMOS check sum error”. Such errors may be caused by the battery having become too weak during the unused time, or by the failure of the CMOS RAM. A multimeter can be used to check voltage of the battery to determine its condition. If the voltage measured is too low then the battery should be changed. If the battery is not changed in a timely manner it can leak and pollute the motherboard. However, the foregoing method is wasteful of time and many users may not associate the appearance of error messages with a weak battery. Moreover, by the time the battery is changed according to the scenario above, the battery has already become too weak, so the user has to reset the CMOS parameters.
Currently a warning circuit exists in the motherboard of the computer. The user can diagnose a problem by recognizing a ringing during a power on self test (POST) of the computer. If some failure such as a loose-contact memory, a keyboard controller error, or a loose-contact graphics card happens, the warning circuit makes a warning noise of a predetermined duration to help the user diagnose a problem according to a technical reference manual.
What is needed, therefore, is a weak battery warning circuit which is able to give a warning before the battery becomes weak.